In the field of telecommunication lines, optical fibers have been relatively recently introduced, and a modulated light signal is input to the lines. Such optical systems are particularly convenient since optical fibers can transmit a signal over long distances with very reduced attenuations.
In order to further increase the signal transmission distance, optical amplifiers are already known which are provided with a portion of the so-called "active" fiber into which light "pumping" energy of a lower wavelength than the signal energy is fed. Such pumping energy causes, inside the active fiber, a transition to the state of laser light emission of the dopants present therein. The presence of a signal having a wavelength corresponding to said laser light emission state gives rise to a decay of the dopant atoms from the laser state to a base state, which is associated with a light emission consistent with the signal thereby causing the amplification thereof.
Said optical amplifiers allow a signal amplification to be achieved in the fiber without resorting to electronic apparatus which needs the transformation of the signal from an optical signal to electrical signal, then electrically amplified and again converted to an optical signal thereby introducing into the telecommunication line all the limitations inherent in the electronic apparatus used and in particular, restricting the transmission frequency.
For their operation, optical amplifiers need a source of pumping light of a particular wavelength, lower than the transmission wavelength, which is introduced into the fiber carrying the transmission signal through a dichroic coupler or optical multiplexer and which diffuses within the active fiber with an increasing attenuation of its light power in the fiber length, mainly due to the transfer of energy to the dopants excited in the laser light emission state.
Therefore, the light pumping power, responsible for the amplifier gain, gradually decreases within the active fiber, so that the exploitation of the active fiber properties becomes increasingly lower over the length thereof.
The minimum light power which is required for each portion of the active fiber, so that an amplification gain can be obtained, is designed as the threshold power, above which a population inversion takes place, that is, there is a higher number of atoms in the excited laser light emission state than in the base state and, therefore, the signal photons can cause a transition from the laser light emission state to the light emission base state thereby giving rise to a gain.
On the other hand, when the light power is lower than the threshold power, the atomic population is higher in the base state and the signal photons are very likely to cause, by themselves, a transition to the excited state, so that a signal attenuation occurs instead of a gain.
Since there is also the possibility of spontaneous decays from the excited state to the base state with a light emission independent of the signal, defined as "noise", in the presence of low gains, that is, with a pumping power slightly higher than the threshold power, there is a low signal-to-noise ratio which impairs the transmission quality. In fact, when the pumping power values are near the threshold values, that is, in a condition of reduced population inversion, a great number of atoms are subjected to spontaneous decay with respect to those in which a stimulated transition giving origin to a gain occurs. As a result, therefore, there is a worsening in the signal-to-noise ratio.
Because of this phenomenon, the active fiber is selected to have a length significantly lower than the length at which the pumping power reaches the level of the threshold power at its end portion.
However, as a result, part of the pumping power is unused so that the amplifier efficiency is restricted, and in addition, since this power diffuses in the transmission fiber downstream of the amplifier, it can give rise to disadvantages, in particular, when the amplifier is a preamplifier at the end of a transmission line and is connected to receiving electronic apparatus.
Therefore, it is desirable to provide an optical amplifier having a reduced "noise figure", that is, having the maximum output signal-to-noise ratio and capable of eliminating the unused pumping power at the amplifier output.